This invention relates to a printhead and a printing apparatus using the same and, more particularly, to a printhead which performs printing by using the ink-jet method and a printing apparatus using the printhead.
Conventional printing methods adopting the ink-jet method have advantages that it is possible to minimize noises made during a printing operation to the ignorable level as well as to perform the printing operation at high speed, furthermore, to fix ink to print on so-called ordinary paper without performing any special processing, thereby generating interest.
Among the conventional printing methods, the ink-jet printing methods, disclosed in the Japanese Patent Laid-Open No. 54-51837 and the German Patent Publication (DOLS) No. 2843064, have unique characteristics different from other printing methods adopting common ink-jet printing methods in the point that thermal energy is applied to liquid to obtain motive power for discharging ink.
More specifically, according to the printing method disclosed in the aforesaid patent publications, printing is performed in such a manner that liquid which is applied with thermal energy changes its state accompanied by sudden volume expansion, and ink droplets are discharged from orifices at the end of a printhead of a printer, caused by action of the state change, then the discharged ink droplets stick to recording medium, such as printing paper.
FIGS. 10A to 10G show a principle of discharging an ink droplet from a printhead in the ink-jet printing method.
In the stationary state, as shown in FIG. 10A, ink 31 filling a nozzle 32 is in a state where the surface tension of the ink at an orifice is in equilibrium to external pressure. In order to discharge the ink under this condition, first, an electric current is supplied to an electrothermal transducer 30 which is in the nozzle for causing a rapid rise in temperature to the ink in the nozzle over a film boiling temperature. Accordingly, as shown in FIG. 10B, the ink 31 neighboring to the electrothermal transducer 30 is heated up and tiny bubbles are created, then the heated portion of the ink vaporizes, thus reaching the film in boiling state. As a result, a bubble rapidly grows as shown in FIG. 10C.
When the bubble grows to the maximum as shown in FIG. 10D, an ink droplet is forced out from an orifice of the nozzle. Then, after stopping the supply of electric current to the electrothermal transducer 30, the grown bubble cools down in the nozzle and shrinks as shown in FIG. 10E. As described above, an ink droplet is discharged from the orifice by growth and shrinkage of the bubble. The size of the ink droplet 33 can be controlled by electric current supply time and sequence to the electrothermal transducer 30.
Further, as shown in FIG. 10F, the ink adjacent to the surface of the electrothermal transducer 30 is rapidly cooled down, and the bubble disappears or shrinks to an ignorable volume. As the bubble shrinks, ink is provided from a common ink chamber to the nozzle caused by the capillary phenomenon, as shown in FIG. 10G, and ready for the next current supply.
Therefore, by reciprocally moving a carriage, loaded with such a printhead (the moving direction of the carriage is referred to as "main scanning direction", hereinafter) and discharging an ink droplet from the nozzle caused by supplying an electric current to an electrothermal transducer in response to image signals generated in synchronization with the carriage movement, an ink image is printed on recording medium, such as printing paper.
In the principle as described above, an ink droplet is formed in accordance with an image signal, thereby an image is printed on recording medium.
Especially, according to the ink-jet printing method disclosed in the DOLS No. 2843064, it can be very effectively applied to the so-called drop-on-demand printing method and a full-line printhead including a plurality of high-density aligned orifices can be easily manufactured, thereby a high-resolution and high-quality image can be obtained at high speed.
The printhead of a printer used in the aforesaid printing method comprises an orifice for discharging an ink droplet, a heat conductor, commonly connected to the orifice, which applies thermal energy causing the discharge of the droplet to liquid, and a printhead board including an ink discharging unit having a flow channel and an electrothermal transducer as means for generating thermal energy.
The above printhead board is provided with a plurality of heaters aligned, drivers corresponding to the plurality of respective heaters for driving corresponding heaters on the basis of the input image data, a shift register for temporarily storing the same number of bits of image data as the number of the heaters so as to output the sequentially inputted image data to each driver as parallel data, and a latch circuit for temporarily storing the data outputted from the shift register, all of which are arranged on a single substrate. The printhead board integrated on the single substrate, as described above, is configured in such a manner that a heating element is made on an IC constructed with bipolar transistors (Bi-CMOS), C-MOS, and the like, and a plurality of such ICs are integrated on a silicon substrate.
FIG. 11 shows a logical circuit of the conventional printhead board comprising 64 printing elements. In FIG. 11, reference numeral 101 denotes the 64 aligned heaters; 102, power transistors; 103, a latch circuit; and 104, a shift register. Further, reference numeral 105 denotes a clock signal input terminal for inputting a clock signal used for activating the shift register 104; 106, an image data input terminal; 107, a strobe input terminal for inputting a heat pulse width control signal used for controlling "ON" period of a power transistor 102 from outside; 108, a logic circuit power supply terminal; 109, a GND terminal of the logic circuit 108; 110, a heater driver power supply (VH) terminal; 111, a GND terminal of the power transistors 102; 117, a latch signal input terminal; and 118, an AND circuit.
In the printhead adopting the ink-jet method, by reducing the number of elements to discharge ink concurrently, the quality of a printed image is not affected by ink supply from an ink tank, thereby high quality image printing can be realized. Therefore, the plurality of printing elements provided inside of the printhead are divided into a plurality of blocks so as to operate each block at different timing and be controlled so that neighboring printing elements do not discharge ink concurrently.
For instance, in the printhead shown in FIG. 11, the 64 printing elements are divided into 8 blocks each of which includes 8 printing elements, and printing operation is performed by each block. Further, among the printing elements in the same block, odd-numbered printing elements and even-numbered printing elements operate at different timing so that the neighboring printing elements do not discharge ink concurrently. Therefore, input terminals 114 to 116 for inputting block selection signals, an input terminal 112 for inputting an odd-numbered element selection signal; an input terminal 113 for inputting an even-numbered element selection signal, and a decoder 119 are provided. Here, the odd-numbered element selection signal is a control signal for selecting the heaters numbered by odd-numbers among numbers put on the left upper position of each heater, 1st, 2nd, 3rd, . . . , 64th, in FIG. 11, to discharge ink, and the even-numbered element selection signal is a control signal for selecting the odd-numbered heaters to discharge ink. Further, the decoder 119 selects one of the 8 blocks in accordance with the block selection signals inputted from the terminals 114 to 116.
In the printer including the printhead having a configuration as described above, image data is inputted from the image data input terminal 106 to the shift register 104 in serial, and, when 64 bits of image data are inputted, the input image data is latched by the latch circuit 103. Then, either the odd-numbered or even-numbered power transistors 102, corresponding to AND circuit 118 which are inputted with the "ON" latched data and the "ON" heat pulse width control signal inputted from the input terminal 107, which are in a block selected by the decoder 119 are turned ON, the heaters 101 are driven, and ink in the flow channels corresponding to the driven heaters 101 is heated up, then the ink is discharged from orifices thereby image is printed.
FIG. 12 is a cross-sectional view of the printhead board shown in FIG. 11.
As shown in FIG. 12, a dopant, such as As (arsenic), is doped to a silicon substrate 201 of a P electronic conductor by ion implantation and diffusion method, thereby forming n type epitaxial layer 203. Further, the n type epitaxial layer 203 is doped with impurity, such as B (boron), thereby forming a p type well region 204. Thereafter, doping is repeated by using methods, such as photolithography, oxidation diffusion, and ion implantation, and p-MOS 250 is formed in the n type epitaxial region and n-MOS 251 is formed in the p type well region. Each of the p-MOS 250 and the n-MOS 251 consists of gate wiring 215 made of polysilicon which is deposited in chemical vapor deposition (CVD) method separated by a gate insulation film 208 of some hundred .ANG. (angstrom), an n or p type doped source region 205, and a drain region 206.
The latch circuit 103 and a logic part of the shift register (S/R) 104 are constructed with the aforesaid MOS transistors.
Further, npn type power transistor 252, constructed with a collector region 211, a base region 212, an emitter region 213, and the like, acted as the driver 102 for the heater 101 is formed in the n epitaxial layer by doping, diffusion process, or the like.
Further, an oxide film separating region 253, formed by field oxidization, separates each element. This field oxide film serves as a heat storage layer 214 in the first layer under a heating element 255. This field oxide film serves as a heat storage layer 214 in the first layer under a heating element 255.
After each element is formed, phosho-silicate glass (PSG) or BPSG is deposited according to the CVD method to form an interlayer insulation film 216, and smoothed by using heat process. Thereafter, wiring is arranged by using an aluminum electrode 217 in the first layer through a contact hole. Then, an interlayer insulation film 218 of silicon monoxide (SiO), for example, is deposited by a plasma CVD method, further, a heater layer 219 is connected to the aluminum electrode 217 through a contact hole.
The passivation film 221 is made of monosilicon mononitride (SiN) film formed by the plasma CVD method. In the upper most layer, anti-cavitation film 222 of tantalum (Ta), or the like, is deposited with an opening part, i.e., a pad part 254. Further, reference numeral 220 denotes another aluminum electrode.
In the above explanation, bipolar transistor is used as the power transistor, however, a MOS transistor can be used also.
As described above, the printhead drivers adopting the ink-jet method aimed for high quality printing are divided into a plurality of blocks as shown in FIG. 11, thereby it is unnecessary to supply ink in a timewise concentrated manner from the ink tank to the printhead, and the ink is supplied in an unstrained manner. Further, by preventing droplets from being discharged from neighboring nozzles simultaneously, stable ink supply from the ink tank to ink nozzles becomes possible. Furthermore, in discharging ink by driving heating elements, since pressure is applied not only in the discharging direction but also in the direction to a common ink chamber 305 (shown in FIG. 13), nozzles neighboring to the discharging nozzles are efficiently refilled with ink, thereby ink supply can be stabilized.
FIG. 13 is a perspective view illustrating a structure of a printhead adopting the conventional ink-jet method. In FIG. 13, reference numeral 301 is a flow channel wall for separating nozzles; 302, a flow channel; 303, a top board; 304, an orifice; 305, a common ink chamber for supplying ink to a plurality of nozzles; and 306, an ink supply tube for supplying ink from an ink tank (not shown) to the common ink chamber 305.
With the aforesaid configuration, actual printing is performed by heating the ink inside of a nozzle by supplying energy to a heating element, making an ink droplet discharged from the nozzle, then fixing the ink droplet on recording medium.
When bubbles are formed and grow in the ink supply tube 306 for supplying ink from the ink tank to the nozzles and in the common ink chamber 305, and move and reach the flow channel 302 as it is refilled with ink, ink may not be discharged even though there is ink in the ink tank. This phenomena is called "ink discharge failure", and some dots are not printed in the actual printing process, thereby resulting in noticeable degradation in printing quality.
Possible reasons why these bubbles are produced would be: (1) relating to a principle of discharging an ink droplet, when ink is suddenly cooled down, a shrunk bubble does not disappear completely and remains; (2) gas dissolved in the ink appears in the common ink chamber 305 as bubbles; and (3) external air enters to the flow channel from the orifice.
Print failure caused by the ink discharge failure results in another printing operation, wasting time for printing and also wastes recording medium. Therefore, the ink discharge failure is unwelcomed in a printing apparatus adopting the ink-jet method.
Accordingly, in the conventional printing apparatus adopting the ink-jet method, the following two countermeasures are taken in order to prevent the above printing failure.
(1) Perform an automatic recovery operation to periodically remove such ink from nozzles before starting a printing operation.
(2) Measure the internal temperature of the printhead by using a thermosensor provided inside of the printhead, detect abnormal rise in temperature inside of the printhead when the ink discharge failure occurs, and perform ink suction recovery in accordance with the detected result. More specifically, the temperature inside of the printhead is measured by the thermosensor before and after an ink discharge operation, so-called preliminary discharge, for discharging viscous ink inside of the nozzles in advance to a printing operation when the printhead has been left unused for a while is effected. Then, whether the ink discharge failure would occur or the ink would be normally discharged is determined on the basis of the difference between the measured temperatures. If the ink supply is stopped in the middle of a printing operation, the internal temperature of the printhead increases by more than 10.degree. C. comparing to the temperature when the ink is normally discharged. Thus, in a case where such the abnormal rise in temperature is detected, the ink suction recovery operation is automatically performed. Further, temperature is measured periodically during performing a printing operation so as to check an abnormal temperature rise in the printhead.
In the aforesaid embodiments, however, there are following problems.
(1) Since ink is periodically sucked from nozzles, ink is sucked even when the ink discharge failure has not occurred. Thus, a considerable amount of ink not actually used for printing is wasted, thereby the running cost per printing paper sheet increases. Further, the sucked ink is stored in a disposal ink tank in the printing apparatus, and this disposal ink tank prevents down-sizing. Especially, in a case where the sucked ink is stored in the disposal ink tank as wasted ink, it is necessary to change or empty the disposal ink tank frequently.
(2) Function for measuring an internal temperature of the printhead and an arithmetic operating function are necessary in the printing apparatus, and the measuring function causes an increase in manufacturing cost. Further, frequent temperature measurement and calculation for obtaining a difference in temperature result in putting a considerable load on the CPU of a control circuit, which decreases printing speed. This is a fatal problem for the printing apparatus. In addition, the ink discharge failure is not detected perfectly.